This invention relates to time-temperature indicators and is particularly concerned with devices which indicate exposure time above a threshold temperature.
For at least the past forty years there has been a desire for a simple means to determine whether an article's exposure to a temperature at or above a predetermined threshold has exceeded a predetermined time limit. Such information is of interest, for example, in determining whether frozen foods have thawed, and much of the prior art has been concerned with this problem. One threshold temperature indicating device, operating substantially independent of time, is shown in U.S. Pat. No. 2,269,035, where a colored paper is provided with an opaque coating of a powdered sharp-melting compound dispersed in a polymeric carrier. Exposure to a temperature above the melting point fuses the compound, rendering it transparent and permitting the colored backing to be seen. Other devices, taking both time and temperature into account, have been based on the fact that a low-melting solid will, upon melting, wick along a porous substrate such as filter paper, cardboard, etc., at a predictable rate, the distance of travel thus being directly related to the time the substance was in liquid form. Illustrative devices incorporating this general principle are shown in U.S. Pat. Nos. 3,414,415, 2,560,537, 2,782,749, 3,118,774, and 3,243,303.
Another time-temperature indicating device, shown in U.S. Pat. No. 3,065,083, is a two-compartment envelope containing a colorless indicator dye separated from a solid fatty acid by a fat-soluble isobutylene: styrene barrier film. When the envelope is heated to a sufficiently high temperature, the acid melts, slowly dissolves the film, and reacts with the indicator dye, causing a color change.
In more recent times, modern technology has imposed new requirements for time-temperature indicating devices. For example, the so-called "fast food" chains, which specialize in the mass production of specific food items, pride themselves on making certain that the food is served while it is still fresh. Thus, a freshly prepared hamburger sandwich is placed in a heated holding area, and, unless it is sold within 10 minutes, it is thrown away. It is difficult for a busy worker to remember when each sandwich has been placed in the holding area, and there thus exists a desire for an inexpensive means to indicate visually the aging of each individual sandwich.
It is believed that none of the time-temperature indicators of the prior art is suitable for the use described in the preceding paragraph, all such prior art devices being more complicated and expensive than the use will permit, unsuitable for the temperature range involved, lacking in customer appeal, incapable of reacting within a brief but finite time span, etc.